Grace Githuthwa heard the attackers before she saw them. They were singing war songs, running from two sides towards the church compound where she and 200 others were sheltering from outbursts of ethnic violence. She grabbed her four children and ran inside the Kenya Assemblies of God Pentecostal church. The hundreds of youths from the Kalenjin tribe armed with bows and arrows and machetes easily overpowered the few Kikuyu men and turned on the women and children.

"They started cutting the church door with a panga [machete]," Githuthwa said. "They were from around here, and even knew some of our names. We kneeled down and surrendered. It was quiet, as we were all praying. We knew this was the end."

Mattresses soaked with paraffin were pushed through the windows and used to block the door. Matches were thrown in.

As the fire engulfed the wooden building, the women grabbed their children and jumped through the burning windows. Githuthwa pushed her two elder children out of the window, and then climbed out holding her three-and-a-half year-old daughter, Miriam, in her arms.

The Kalenjin youths were still waiting, "cutting people like firewood" as they emerged. "They snatched Miriam from me and threw her back into the fire," said Githuthwa, as she returned to the church, near Eldoret in western Kenya, hoping that by some miracle Miriam had survived.

Smoke was still rising from the embers. A dozen blackened bicycles were stacked neatly against what had been the wall of the church. Tin cups were strewn across the ground. There was a child's shoe, a woman's sandal, a bible. In the small cooking hut alongside the church, burnt but not completely destroyed, lay corncobs and beans that were being prepared for lunch when the attack started.

In the far corner of the church were three bodies. They were charred beyond recognition, all apparently children. They lay on their sides. As policemen stood guard, five Red Cross workers wearing surgical gloves and facemasks moved the bodies on to blankets. Soon there were 12 corpses lying side by side, all but one of them children, a few of them babies. One of them was probably Miriam. Her mother broke down in tears.

Two blankets, one brown, one purple, were taken from the belongings strewn across the compound and laid over the bodies.

The search continued in debris at the far end of the church. Another body soon emerged. Another child. On the road outside the church compound, flanked by tall cypress trees, lay two more corpses. A man in a suit was spreadeagled on his back. On the side of his head was a gaping machete wound. Next to him was a woman with grey hair. There were slash marks on her torso. In the cornfield 50 metres away lay two more bodies, one a man with a leg disfigured by polio, who was partly burned. There were 17 bodies in all; there could have been more nearby.

A second woman approached the church. Margaret Muthoni, 38, was looking for her six-year-old niece, Miriam Ngendo. "I was carrying her out of the church, but she fell," Muthoni said. "I had my six children with me and we had to run for safety. I could not go back for her."

She walked over to the bodies and lifted one of the blankets. Then she began to scream, a terrible, grief-laden scream, and dropped to her knees.

A few miles away, the road was littered with obstructions every few hundred metres: trees, telephone poles and large rocks forced cars on to the verge, where youths with clubs and knives were sitting. At Ngeria Junction, hundreds of angry youths and men, all from the Kalenjin ethnic group, were gathered.

They said they felt cheated by the election, awarded in dubious circumstances to President Mwai Kibaki over opposition leader, Raila Odinga. They wanted revenge, and it was Kibaki's Kikuyu ethnic group who were going to suffer. Asked if they knew about the church massacre, all the youths nodded.

"We were there," said one man, who said his name was Patrick. "We got message that the Kikuyus were arming near the church. So we went to give reinforcements to the Kalenjins there."

Another man carried on: "The men and women had babies and small children, but they carried pangas to defend themselves. Is someone with a panga innocent? It is not our custom to kill women and children. We told them to come out of the church, but they locked the door and refused to come out. So we burned them."

A third youth spoke. "They were not worshipping in the church. They were hiding. That makes it a cave not a church. Let Kibaki send a plane for the Kikuyus. They can go ... or they will be killed."

Several more men confirmed that youths from this village had helped carry out the attack.

The fear and confrontation extended across much of the Rift Valley region. Baraton, a young Kikuyu student from the University of East Africa, spoke via a mobile phone. She could not leave her room, she said. Since election day, Kalenjin youths, some of them her classmates, had started threatening all the Kikuyus and Kisiis - also accused of supporting Kibaki - on campus. A gift of a cow and then a bull had satisfied them for a day or two. But they had started fires outside the main gate, and were demanding identification cards from anyone passing through.

"We desperately need the police to come and protect us," she said.

The fear cut across ethnic lines; most Kalenjins had nothing to do with the violence, and fear of reprisals was growing. Moses, a Kalenjin in the Nandi Hills, sent a text message: "No transpot. Road blocked with stons. Electrisity disconnected. No car fuel. Houses still baning and robary. We r so scared."

At the New Heuvel petrol station in Cheptiret, telephone poles were laid across the road. Dozens of men stood around, many of them holdings bows with quivers full of metal-tipped arrows slung across their backs. Suddenly they pulled back, crouching in ditches and behind walls, their bow-strings taut, only desisting when the approaching police detail announced its peaceful intention.

As a few of the Kalenjin men approached the vehicle, Chief Inspector Salesiho Njiru said: "These people don't need a harsh tone. We are just going to try to negotiate a way through."

Rugut Brigen, an assistant university lecturer, told Njiru that the bodies of two Kikuyu men lay beside a burnt-out minibus a few hundred metres on. They were killed on Monday, and Brigen wanted police to remove the bodies. "The people stoned them when they did not slow down for our roadblock," he said. "They could not control their anger at the election result."

In front of the minibus, several truck trailers blocked the way. On either side of the road were more than 1,000 armed men, who occasionally broke into a war song. Keeping them in check was their leader, "Michael", who works for an international aid agency. At a single shouted word everyone sat down.

"We are not going to have a ceasefire until the true results are announced," he told the police. Nobody should try to stop them until then, he said. The district officer, a Luo from Odinga's ethnic group, had been killed by a bow and arrow the day before after shooting a boy in the leg while he was putting up roadblocks.

"Today, it is bows and arrows. In three days, if Kibaki has not resigned, we will have guns from Uganda," said Michael.

Several army vehicles arrived, and after tense negotiations, it was agreed that they could escort a long line of vehicles through the town, driving slowly as the Kalenjin men, their clubs, knives and bows at their sides, looked on.

Among those fleeing by car was Moses Maina, 36, a Kikuyu. He had already sent his wife and children by air to Nairobi after chartering a plane with several other families. "I was born in Eldoret," he said, glancing nervously towards a burnt-out minibus. "My father came here in 1950. This is my home, and now I am are running away from it. Where am I supposed to go."

Michael said that was not his problem. "The Kikuyus were treated like guests in the Rift Valley, but Kibaki let them down. It is over. We can never trust them again. We will never let them back," he said.